Fusion transcript analysis reveals slower response kinetics than multiparameter flow cytometry in childhood acute myeloid leukaemia

INTRODUCTION: Analysis of measurable residual disease (MRD) is increasingly being implemented in the clinical care of children and adults with acute myeloid leukaemia (AML). However, MRD methodologies differ and discordances in results lead to difficulties in interpretation and clinical decision‐making. The aim of this study was to compare results from reverse transcription quantitative polymerase chain reaction (RT‐qPCR) and multiparameter flow cytometry (MFC) in childhood AML and describe the kinetics of residual leukaemic burden during induction treatment. METHODS: In 15 children who were treated in the NOPHO‐AML 2004 trial and had fusion transcripts quantified by RT‐qPCR, we compared MFC with RT‐qPCR for analysis of MRD during (day 15) and after induction therapy. Eight children had RUNX1::RUNX1T1, one CBFB::MYH11 and six KMT2A::MLLT3. RESULTS: When ≥0.1% was used as cut‐off for positivity, 10 of 22 samples were discordant. The majority (9/10) were MRD positive with RT‐qPCR but MRD negative with MFC, and several such cases showed the presence of mature myeloid cells. Fusion transcript expression was verified in mature cells as well as in CD34 expressing cells sorted from diagnostic samples. CONCLUSIONS: Measurement with RT‐qPCR suggests slower response kinetics than indicated from MFC, presumably due to the presence of mature cells expressing fusion transcript. The prognostic impact of early measurements with RT‐qPCR remains to be determined.